The present invention relates to composites made from at least one layer of foam material and another layer of either foam or film material. Such composites are currently made by lamination. For example, a layer of polyethylene film will be laminated with an adhesive to a layer of polystyrene foam in order to create a foam composite product which has greater board strength than would a foam layer alone.
Such laminating operations are expensive. Large sheets of polymeric material have to be stored, moved to an assembly area, coated with adhesives and laminated together. Handling can become especially difficult when laminating three or four layers of polymeric foam or foam and film together.
Composites of different films are often made by coextrusion. Coextrusion greatly reduces handling problems and expense, since the films which are to be joined in a composite are simultaneously extruded from the base resin. The only large sheets which have to be handled are the finished product.
Attempts have been made to adapt known coextrusion processes to the production of composite structures comprising at least two distinct layers, at least one of which comprises a foamed layer. For example, U.S. Pat. No. 3,557,265 discloses the coextrusion of film layers, foam layers and foam-film layers with the foam layers being extruded from a feed comprising an expandable thermoplastic resin, i.e., the resin contains blowing agents when originally heat-plastified. Thus, the '265 patent feeds a first polymer containing a blowing agent into a heater/extruder where it is melted and conveyed to a feedblock and die and interdigitated with another polymer melt stream. It is difficult to control process temperatures during the heat plastification of the polymer-blowing agent feed without some foaming taking place. Generally, the attempts to extend coextrusion technology to include coextrusion of foam-foam or foam-film layers have given mixed results. While commercially acceptable composites can be produced by coextrusion, the method is not consistently reproducible with the same production run giving not only acceptable product but also significant amounts of unacceptable product which must be scrapped. In addition, the coextruded composites so obtained quite often have a wavy, corrugated appearance instead of the more eminently desired smooth outer surface. Notwithstanding such problems, there is a strong desire to develop coextrusion of foam-foam and foam-film composites because of the significant cost savings which coextrusion promises.